linux-kernel - Re: [PATCH v3 3/3] x86: usercopy: reimplement arch_within_stack

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Date:   Tue, 10 Apr 2018 10:35:38 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "'kpark3469@...il.com'" <kpark3469@...il.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "james.morse@....com" <james.morse@....com>,
        "keun-o.park@...kmatter.ae" <keun-o.park@...kmatter.ae>,
        "psodagud@...eaurora.org" <psodagud@...eaurora.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/3] x86: usercopy: reimplement
 arch_within_stack_frames with unwinder

On Mon, Apr 09, 2018 at 01:14:58PM +0000, David Laight wrote:
> From: kpark3469@...il.com
> > Sent: 09 April 2018 12:59
> >
> > The old arch_within_stack_frames which used the frame pointer is
> > now reimplemented to use frame pointer unwinder apis. So the main
> > functionality is same as before.
> 
> How much slower does this make the code?
> Following stack frames using %bp is reasonably quick.
> I can't imagine some of the other unwinder APIs being any where
> near that fast.
> While fine for fault tracebacks, using them during usercopy
> is likely to have measurable performance impact.

Agreed, using the unwind interface is going to be quite a bit slower
than the current manual approach.  So this patch has only drawbacks and
no benefits.

The only benefit would be if you used the unwind API in a generic
manner, such that it also worked for the ORC unwinder.  But even then I
think we'd need to see performance numbers, with both FP and ORC, to see
how bad the impact is on usercopy.

-- 
Josh